rpc: set Request.GetBody for client requests (#24292)

When talking to an HTTP2 server, there are situations where it needs to
"rewind" the Request.Body. To allow this, we have to set up the Request.GetBody
function to return a brand new instance of the body.

If not set, we can end up with the following error:

    http2: Transport: cannot retry err [http2: Transport received Server's graceful shutdown GOAWAY] after Request.Body was written; define Request.GetBody to avoid this error

See this commit for more information: cffdcf672a?visible=2

rpc/http.go

@@ -181,6 +181,7 @@ func (hc *httpConn) doRequest(ctx context.Context, msg interface{}) (io.ReadClos
 		return nil, err
 	}
 	req.ContentLength = int64(len(body))
+	req.GetBody = func() (io.ReadCloser, error) { return ioutil.NopCloser(bytes.NewReader(body)), nil }
 
 	// set headers
 	hc.mu.Lock()